This invention relates to a melt-extrudable thermoplastic polyolefin-based composition which when extruded into films, fibers, nonwoven fabrics or composites, results in a material or nonwoven fabric which exhibits durable wettability.
Polyolefins, especially polypropylene, are used in large quantities to make nonwoven fabrics and films. Polyolefin nonwoven fabrics, such as carded webs, spunbond, meltblown or composites thereof, are preferred as components in sanitary articles, such as single use diapers, feminine hygiene products and incontinence care products. The recognized benefits of polyolefin based, especially polypropylene, fabrics include the relatively low raw material cost, ease of manufacturing, desirable strength to basis weight ratio and softness.
Sanitary articles generally contain an absorbent core component of materials capable of absorbing several times their weight in liquids. Usually the article includes at least one outer covering or lining which contacts the user's skin on one side of the core and the an exterior layer contacting the environment on the other side of the core. Softness and liquid permeability are required of fabrics used for the inner linings. The liquid permeability should take the form of allowing liquid to pass through the fabric and into the inner absorbent core, while not actually absorbing fluids in the process. An additional desired feature is for the inner lining, i.e. the cover sheet for the absorbent core, to remain fluid permeable even after extended wear and repeated insults of fluid, such as routinely occurs with infant diapers in situ. Another highly desired, but difficult to provide, feature of inner lining fabrics is that they resist having liquids collected in the absorbent core bleed back through to the user's skin when pressure is applied--such as an infant sitting in a wet diaper.
Nonwoven fabrics and composites made of cellulosic materials pass and absorb liquids even after repeated insults, but they do not routinely resist the flow back of the retained fluids under pressure. Thermoplastic fibers, such as polyesters and polyolefins have already been described as being preferred for these end uses for economic, aesthetic and strength reasons. However, polypropylene is, by its nature, hydrophobic. When spun into fibers or filaments which are used to form a fabric, the resulting fabric is also hydrophobic or non-wettable. Thus, the fabric must be specially treated or altered in some way to render the fabric wettable, that is, able to allow the passage or transfer of fluids, if the fabric is to be suitable for use as an inner lining fabric for a sanitary article.
For purposes of clarification, it should be noted that absorption indicates that the material actually swells with added water. In contrast, wettability, such as used herein, denotes a change in surface tension that permits a layer of water to form on the surface of a solid, such as a fiber, for the purpose of facilitating the movement of the liquid flow past or through the wettable material.
It is known in the industry that certain surfactants, such as Triton X-100 from Rohm and Haas, can be applied as an aqueous solution or suspension to the surface of hydrophobic fibers, filaments or nonwoven fabrics with the resulting effect of rendering the fibers, filaments or fabrics wettable, although not absorbent. These topical treatments can be applied by any means familiar to one skilled in the art, such as foaming, spraying, dip-and-squeeze or gravure roll. In almost every case, some sort of heating step is required to remove residual water or solvents used to prepare the surfactant solution or suspension. This step adds significantly to the manufacturing costs and complexity. Further, thermoplastics are altered by exposure to heat and careful monitoring of the heating process is required to ensure that fabric properties are not adversely affected. Further, the surfactants are not strongly chemically bonded to the fiber or filament surfaces, such topical treatments are not durable. They tend to wash off during repeated fluid insults or rub off during use.
In an effort to correct this deficiency, corona discharge treatments have been used to alter the electrochemical potential of the surfaces of fibers or filaments. The effect is to render surfaces more reactive with the result that hydrophobic surfaces become more wettable. However, these electrical potential changes are also not permanent, being particularly subject to environmental effects, such as storage in moist environments.
An additional advancement is the use of surface chemical treatments where the surfactants are covalently bonded to the polymer.
Another approach is the incorporation of chemical agents in the thermoplastic polymer before it is extruded into fibers, filaments or nonwoven fabrics. Agents, such as siloxanes, have been proposed for this purpose. Here, the object is to impart a durable change in the wettability of the fibers or filaments. The performance model theory states that the melt additives become dispersed in the molten polymer and are bound in the matrix when the polymer cools during fiber or filament quenching. Over time, or due to the effects of further processing, the additive rises to the surface of the fibers or filaments, a phenomenon called blooming, imparting durable wettability.
Fatty acid esters have been used as fabric softener compounds, such as described in U.S. Pat. No. 5,593,614. The melt addition of a di-fatty ester to polyolefins is described in U.S. Pat. No. 5,439,734 to Kimberly-Clark. The melt addition of this di-acid ester was described as imparting wettability durable up to three fluid insults.
Polyethylene glycols esters (PEG esters) have been recognized as useful in the preparation of hydrogels and wettable membranes, directed towards wound care, as described in U.S. Pat. Nos. 5,700,286 and 5,698,074. PEG esters have also been used to topically treat hydrophobic fibers, as described in U.S. Pat. No. 4,073,993.